RNA binding by Periphilin plays an essential role in initiating silencing by the HUSH complex. RNA binding by Periphilin plays an essential role in initiating silencing by the HUSH complex

The human silencing hub (HUSH) complex is a transcription-dependent, epigenetic repressor complex that provides a genome-wide immunosurveillance system for the recognition and silencing of newly-integrated retroelements. The core HUSH complex of TASOR, MPP8 and Periphilin, represses these retroelements through SETDB1-mediated H3K9me3 deposition and MORC2-dependent chromatin compaction. HUSH-dependent silencing is RNA-mediated, yet no HUSH components encode any RNA-binding domain. Here we used an unbiased approach to identify which HUSH component was able to bind RNA and determine whether RNA-binding was essential for HUSH function. We identify Periphilin as the major RNA-binding component of the HUSH complex and show that Periphilin’s N-terminal domain is essential for both RNA binding and HUSH function. Periphilin binding to RNA was independent of its interaction with TASOR or MPP8, as its N-terminal domain was sufficient for RNA targeting. The artificial tethering of Periphilin, to a HUSH-insensitive, nascent transcript, enabled the HUSH-dependent silencing of the transcript. This tethering of Periphilin allowed the RNA-binding region of Periphilin to be removed such that only its C-terminal domain was required, for oligomerisation and interaction with TASOR. We therefore show that Periphilin is the predominant RNA-binding protein of the HUSH complex and this RNA-binding is essential for HUSH activity. Overall design: To characterize the endogenous targets of PPHLN1 binding we initially used UV-crosslinked RNA immunoprecipitation sequencing (UV-RIPseq) in 293T PPHLN1 and SETDB1 knockout cells, expressing a panel of HA tagged PPHLN1 constructs